As I am sure you are by now well aware, in November I decided to start
releasing details on all vulnerabilities I found in web-browsers that I
had not released before. As I was unable to publish all of them within a
single month, I will try to continue to publish all my old
vulnerabilities, including those not in web-browser, as long as I can
find some time to do so. If you find this information useful, you can
help me make some time available by donating bitcoin to
183yyxa9s1s1f7JBp­PHPmz­Q346y91Rx5DX.

This is the twenty-third entry in the series. This information is
available in more detail on my blog at
http://blog.skylined.nl/20161201001.html. There you can find a repro
that triggered this issue in addition to the information below.

Follow me on http://twitter.com/berendjanwever for daily browser bugs.

MS Edge CMarkup::EnsureDeleteCFState use-after-free
===================================================
(MS15-125, CVE-2015-6168)

Synopsis
--------
A specially crafted web-page can trigger a memory corruption
vulnerability in Microsoft Edge. I did not investigate this
vulnerability thoroughly, so I cannot speculate on the potential impact
or exploitability.

Known affected software and attack vectors
------------------------------------------
* Microsoft Edge 11.0.10240.16384

  An attacker would need to get a target user to open a specially
  crafted web-page. Disabling JavaScript does not prevent an attacker
  from triggering the vulnerable code path.

Description
-----------
At the time this issue was first discovered, MemGC was just introduced,
and I had not yet fully appreciated what an impact it would have on
mitigating use-after-free bugs. Despite MemGC being enabled in Microsoft
Edge by default, this issue appeared to me to have been a use-after-free
vulnerability. However, both Microsoft and ZDI (whom I sold the
vulnerability to) describes it as a memory corruption vulnerability, so
it's probably more complex than I assumed.

At the time, I did not consider this vulnerability to be of great
interest, as there was no immediately obvious way of controlling the
vulnerability in order to exploit it. So, I did not do any further
investigation into the root cause and, if this was indeed a
use-after-free, how come MemGC did not mitigate it?
In hindsight, it would have been a good idea to investigate the root
cause, as any use-after-free that is not mitigated by MemGC might
provide hints on how to find more vulnerabilities that bypass it.

Time-line
---------
* August 2015: This vulnerability was found through fuzzing.
* August 2015: This vulnerability was submitted to ZDI.
* December 2015: Microsoft addresses this vulnerability in MS15-125.
* December 2016: Details of this vulnerability are released.

Cheers,

SkyLined

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